1. Field of the Invention
The present invention relates to an optical device using a waveguide in optical communications, and more specifically to an optical modulator.
2. Description of the Related Art
FIG. 1A shows the configuration of a conventional Mach-Zehnder modulator.
The modulator includes optical waveguides (A) and (B), and coplanar (CPW) electrodes 10 and 11. The electrode includes the signal electrode 11 and the ground electrode 10 on both sides of the signal electrode 11, and an electric polarization inversion area 13 is formed at the central portion. In this portion, an optical waveguide is formed on the +Z plane. A substrate is a Z-cut lithium niobate (LiNbO3), and SiO2 is formed as a buffer layer 12 for suppressing the optical absorption by the electrode between the substrate and the electrode. Although not shown in FIG. 1, it is preferable to form Si film on SiO2 to suppress the temperature drift by electric charge held between the buffer layers because of pyroelectric effect.
The optical modulator using an electro-optical crystal such as a LiNbO3 substrate, etc. is formed by providing a metal film on a part of the crystal substrate, forming an optical waveguide by heat diffusion or proton exchange in a benezenecarboxylic acid after patterning, and providing electrodes near the optical waveguide. At this time, to prevent the optical absorption by the electrode, an insulating film such as SiO2, etc. as a buffer layer is formed between the electrode and the substrate. Normally, an optical waveguide is formed on the −Z plane of crystal because, for example, when a waveguide is formed on the +Z plane in the LiNbO3 crystal, there is an unstable phenomenon of domain inversion occurring on the surface. Such a phenomenon is well known as described in the non-patent document 1, etc. In this example, the +Z plane is a plane uniquely determined by the crystal having spontaneous electric polarization. The spontaneous electric polarization direction is defined as a Z direction, the plane having a waveguide of an electric polarization inversion area in the example shown in FIG. 1 is a +Z plane, and the non-inversion area is a −Z plane. After forming an optical waveguide on the −Z plane, a strong electric field is applied to the substrate, thereby inverting the electric polarization direction of the −Z plane and obtaining a +Z plane.
When an optical modulator is driven at a high speed, the terminals of a signal electrode and a ground electrode are connected by a resistor to obtain a traveling wave electrode, and a microwave signal is applied from the input side. At this time, the refractive indexes of the parallel waveguides (A) and (B) are changed by an electric field into +Δna and −Δnb respectively, and the phase difference between the parallel waveguides (A) and (B) is changed, thereby outputting an intensity-modulated signal light from an output waveguide. By changing the shape of the section of the electrode, the effective refractive index of a microwave can be controlled, and by adjusting the speed between the light and microwave, a broadband optical response characteristic can be obtained. However, since the absolute values of the electric field to be applied to the parallel waveguides (A) and (B) are different so that Δna<Δnb, a phenomenon (chirp) of changing the wavelength of an output light during the transition from the ON status to the OFF status is generated. To solve this problem, the substrate electric polarization inverted in a part of an area is used. A signal electrode is designed to pass on the waveguide (A) in a non-inversion area, and on the waveguide (B) in an inversion area. In FIG. 7, when L1=L2, the light passing the waveguides (A) and (B) respectively change in phase by +Δθs and −Δθg in the non-inversion area, and by +Δθg and −Δθs in the inversion area. The Δθg and Δθs respectively indicates the amount of phase change of the light by the ground electrode 10 and the signal electrode 11. Therefore, the phases of the light passing the waveguides (A) and (B) change in the Y branch waveguide on the output side respectively by +(Δθs+Δθg) and −(Δθs+Δθg), thereby performing a phase modulation with equal absolute values and inverted signs. Therefore, the wavelength chirp can be set to 0. Additionally, the amount of chirp can be adjusted by changing the ratio between the L1 and L2.
FIG. 1A is a top view of an optical modulator. FIG. 1B is a sectional view along the ling A-A′ of the optical modulator shown in FIG. 1A. Since the electric polarization of a substrate is a non-inversion area, the plane on which the optical waveguides (A) and (B) are provided is the −Z plane. That is, the direction of the +Z plane which is the direction of electric polarization is downward. The buffer layer 12 is provided on the optical waveguides (A) and (B) provided on the substrate, and the ground electrode 10 and the signal electrode 11 are provided on the buffer layer 12. FIG. 1C is a sectional view along the line B-B′ of the optical modulator shown in FIG. 1A. Since the electric polarization of the substrate in this portion is an inversion area, the plane on which the optical waveguides (A) and (B) are provided is the +Z plane. That is, the direction of the +Z plane as the direction of the electric polarization is upward. The buffer layer 12 is provided on the optical waveguides (A) and (B) provided on the substrate, and the ground electrode 10 and the signal electrode 11 are provided on the buffer layer 12.
When an optical modulator having the above-mentioned electric polarization inversion structure is used, the +Z plane of the crystal is necessarily used. However, as a result of detailed reliability test, we have found the phenomenon that the operation point of a modulator using the +Z plane greatly changes (changed by several 10V's) by adding a temperature test such as a heat cycle, etc. An operation point of a modulator depends on the phase difference between the parallel waveguides (A) and (B) shown in FIG. 1A, and a shift has a large undesired influence on the transmission characteristic, For a countermeasure against these problems, the techniques described in the patent documents 1 and 2 have been developed.
[Non-patent Document 1] S. Miyazawa, J. Appl. Phys., Vol. 50, No. 7, 1979
[Patent Document 1] Specification of Japanese Patent No. 02873203
[Patent Document 2] Japanese Patent Publication No. H05-078016
However, as an experiment result, the following points have been clearly indicated.
An operation point has changed on the +Z plane.
A change occurs when the +Z plane is used regardless of how performing a producing step.
The problems cannot be solved in the reliability establishing method for the temperature drift of the optical modulator generated by a conventional pyroelectric effect.
The conventional countermeasure against the temperature drift is explained below by referring to FIGS. 2A and 2B.
As shown in FIG. 2A, electric charge is generated on a strong dielectric crystal when a temperature changes. It is referred to as pyroelectric effect. By distributing the electric charge to the buffer layer 12 which is an insulating film asymmetrically about the optical waveguides (A) and (B), the phase asymmetrically changes between the two waveguides by the electric field formed by the electric charge, thereby causing a temperature drift. Thus, a method of symmetrically distributing electric charge by forming a conductive film 15 on the top surface of the buffer layer 12 as shown in FIG. 2B is well known.
However, since the degradation phenomenon found in the above-mentioned experiment cannot be completely solved in the method shown in FIGS. 2A and 2B, a new solving method is demanded.